1. Field of the Invention
The present invention relates to lithium secondary batteries, and more particularly, electrolytes for use with lithium secondary batteries.
2. Description of the Related Art
Lithium metal has been used for negative electrode active materials. However, because of potential dangers of explosion created by the formation of dendrites and short circuits associated with the use of lithium metal, carbon group materials are more commonly used as negative electrode active materials.
The carbon group materials used in negative electrode active materials of lithium batteries typically include a crystalline carbon group, such as natural graphite, or artificial graphite, or a non-crystalline carbon group, such as soft carbon or hard carbon.
The non-crystalline carbon group typically has a large battery capacity, but it has a large irreversibility loss during the battery charge-discharge process. Similarly, the crystalline carbon group, such as graphite, has a high theoretical capacity (i.e. 372 mAh/g), but it still has problems with deterioration.
In addition, the current high theoretical capacities of the existing graphite and carbon group active materials are still not adequate for use in high capacity lithium batteries.
To address these problems, lithium batteries with metal composites have been proposed for use as negative electrode active materials. Examples of metal composite materials include aluminum, germanium, silicon, tin, zinc, lead, and so on.
These materials can be used to produce batteries with high capacity and energy density because the materials themselves have high capacity and high energy density. These metal composite materials can occlude and release lithium ions better than the conventional negative electrode active materials using carbon group materials. For example, pure silicon is reported to have a theoretical capacity as high as 4017 mAh/g.
However, inorganic particles, such as silicon or tin, when included in the negative electrode active material, can cause volume expansion of as much as 300 to 400%, which can become dangerous when the battery is exposed to high temperatures. Further, the inorganic particles can be problematic when lithium ions are released during the charging and discharge process. During this process the inorganic particles contract, thereby causing a volume change to occur and the negative electrode active material to be separated from the negative electrode collector. As a result, the conductivity between the negative electrode active material and the collector may decrease, thereby decreasing the battery capacity and cycle life.